For a demo it does well to demonstrate the core idea of the developer. It is clear to see that it is in very early development but even then, it manages to add a fair range of unique enemy types, weapon types, and an extremely fun boss fight.
Its in game systems are clearly in their infancy but they have potential. The upgrade system in particular as it is fun even now.
There are areas of polish missing such as title screen, proper tutorial and user experience game testing. However, as a first demo made by one person, I do not think this should be judged very harshly.
But things I do feel the game lacks at this stage are doors visible on the mini-map as that would help navigating levels, controls menu in case you forget, and maybe a less excessive use of the time slowdown effect for when the player is hit as it breaks to flow of the game.
Having some furniture in each room and different coloured walls depending on the floor. Would also help to make the game less monotonous as it would add variety to the room layout and differentiate the floors from each other besides just a meta value saying ‘floor 2’ as an example.
Overall, good work, game needs some polishing but has a lot of potential and I would recommend others to give it a try or at least follow the creator to keep up to date on future improvements and additions.